icomplete provides a convenient way to quickly select an element
among the possible completions in a minibuffer.

It is enabled by calling (icomplete-mode 1)

Recently, icomplete-vertical-mode was added.  But calling
(icomplete-vertical-mode 1) does not enable vertical completion,
unless one has called (icomplete-mode 1) first.

It is more natural to simply enable the mode by calling
(icomplete-vertical-mode 1), the same way as one enables
icomplete.  Either call (icomplete-mode 1) or
(icomplete-vertical-mode 1) to activate icamplete with the
appropriate style.